Note that this value is set during print preparation phase (gtk.types.PrintStatus.Preparing), so this value should never be get before the data generation phase (gtk.types.PrintStatus.GeneratingData). You can connect to the gtk.print_operation.PrintOperation.statusChanged signal and call gtk.print_operation.PrintOperation.getNPagesToPrint when print status is gtk.types.PrintStatus.GeneratingData.
This is typically used to track the progress of print operation.